This invention relates to treatment and disposal of sodium nitrite-containing wastewater generated from ship steam boiler maintenance operations, and is particularly concerned with the destruction of the hazardous sodium nitrite in such wastewater, as well as the treatment of toxic heavy metal ions, removal of environmentally objectionable suspended solids, and disposal of sludge also present in such wastewater.
One of the regular maintenance tasks for the Navy's surface ships is cleaning of steam boilers. Wastewater is generated during cleaning of the steam boiler tubes by hydroblast washing. There are two other steam boiler mainteance operations that generate wastewater, namely, (1) hydrostatic tests to check for leaks in boilers before hydroblast washing and (2) boiler lay-up after hydroblast washing and before boiler light off. The water solution used in these operations contains sodium nitrite, a corrosion inhibitor. Thus, the combined wastewater from the above steam boiler cleaning operations is sodium nitrite-containing wastewater. Since the sodium nitrite is not measurably consumed or chemically altered in these cleaning operations, the average nitrite concentration in the wastewater is virtually the same as that in the virgin cleaning solution. The average concentration of sodium nitrite in such wastewater is about 1200 mg/l. However, concentrations of sodium nitrite in the wastewater from the above cleaning operations can be substantially higher, e.g. 1400 mg/l.
Such wastewater is therefore high in sodium nitrite, and also contains suspended solids, sludge, and toxic heavy metal ions. Since the sodium nitrite is the main pollutant of concern, this wastewater is termed "sodium nitrite wastewater".
Although there are no Federal discharge limits to regulate this wastewater, many states have classified such wastewater as hazardous due to the sodium nitrite, heavy metal ions, suspended solids and sludge therein. Some municipal authorities have imposed discharge limits at 33 mg/l for nitrite and 45 mg/l for nitrate. These levels are the same as those in the EPA's drinking water standards. In the case of naval operations, most of the Navy's nitrite wastewater is combined with other wastes at Naval shipyards, with no effective treatment method at hand for the combined wastewater. Disposal is by contract hauling at a high cost per gallon of wastewater. Moreover, the producers of hazardous wastewater, the shipyards, are liable for the safe disposal of such wastewater by contractors.
The main concern with this wastewater is that unmitigated discharge of nitrite, a nutrient, could lead to eutrophication in the receiving waters. Eutrophication results in serious water quality problems, such as taste and odor in water supplies, turbidity, unsightly scums and choking of waterways, and consequently endangers aquatic species such as fish.
Many of the municipal wastewater treatment systems cannot handle high nitrite wastewater, and refuse to accept it.
The use of chemical means to treat nitrite-bearing wastewater is largely presently at the experimental level. In the general literature, for example, few articles are concerned with the chemical treatment of nitrite wastewater, and particularly high nitrite wastewater.
The use of sulfamic acid, NH.sub.2 SO.sub.3 H, for the detoxification of a nitrite-containing wastewater is discussed in German Patent No. 3506825. The patent discloses that nitrite wastewater can be treated with sulfamic acid to generate nitrogen gas and hydrogen sulfate ion. Sulfuric acid, H.sub.2 SO.sub.4, and also sodium hydroxide, NaOH, were added to adjust the pH during the reaction. The patent discloses the use of concentrations of sulfamic acid with respect to sodium nitrite, such as a proportion of approximately 1 to 1 moles of sulfamic acid per mole of sodium nitrite.
In a related article by Y. M. Kostrikin and O. V. Teterina, "Rate of the Reaction of Nitrites with Sulfamic Acid" Energetik, Vol. 10, 1987, page 22, dosage rates and temperature effects were reported for the sulfamic acid and nitrite reaction. In the article it is noted that at 50.degree. C. it would require an excess dosage of 10 times to achieve a 9% nitrite removal, whereas at 70.degree. C. 2.5 times as much sulfamic acid is required.
One object of the present invention is the removal of sodium nitrite from sodium nitrite wastewater produced from steam boiler maintenance operations.
Another object is to treat the above sodium nitrite wastewater in a manner so as to convert the sodium nitrite into an innocuous form.
Still another object is to convert the sodium nitrite in sodium nitrite wastewater to innocuous forms, and also to remove toxic metal ions and suspended solids from such wastewater, to permit discharge of the treated wastewater to sewers.
Still another particular object is to provide an integrated process to treat the hazardous sodium nitrite wastewater completely for discharging directly to sewers without going through the treatment facilities, such as the Navy's industrial wastewater treatment plant, where heavy metal ions removal and sludge reduction are usually carried out.